With the rapid development of the information communications technology, the exchange of voice, image and data information is increasing. Especially as the result of the widespread application of the Internet, people have higher requirements for broadband communication. In a bid to come up with a low-cost and high-quality system to meet people's requirements for broadband communication in the shortest possible time, the wavelength division multiplexing (WDM) technology is widely used.
Another way of increasing the optical fiber transmission capacity is to further narrow the channel spacing. Currently, the channel spacing are all 100 Hz or 200 Hz, according to the provisions of the International Telecommunication Union (ITU). If people want to conduct low-cost capacity expansion of the original system, they prefer an optical comb filter that realizes the expansion by changing the channel spacing without altering the original equipment and system. The existing optical comb filter divides one-channel multi-wavelength optical signals into two channels, one of which have odd-channels of wavelengths and the other of which have even-channels of wavelengths, and the channel spacing becomes twice the original. Currently, a MGTI-type comb filter is widely used, thanks to its low cost and mature technology. However, the MGTI-type optical comb filter usually adopts a GT cavity structure to realize a flat top, but the channel bandwidth is small, so it sees low bandwidth utilization.